1. Field of the Invention
This invention relates to a fixing device for fixing a developer transferred onto a recording material and an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as a laser printer, a general fixing device for fixing a developer transferred onto a recording material includes a cylindrical heating roller and a pressurization roller for coming in contact with the heating roller in parallel. The fixing device allows a recording material such as recording paper to pass through a nip between the heating roller and the pressurization roller and fixes the unfixed developer such as toner transferred to the recording material by heat of the heating roller (for example, heated to a temperature of about 150° C.).
A halogen lamp or an electromagnetic induction heater is used as a source for heating the heating roller.
In a fixing device having a halogen lamp as a source for heating a heating roller, the halogen lamp is placed in a hollow heating roller and an electric current flows into the halogen lamp to thereby radiate infrared rays from the halogen lamp to the inner wall of the heating roller and are converted into heat. The heat is then transmitted to the surface of the heating roller. This configuration requires a supporting device for placing the halogen lamp in the heating roller and connection parts to an electric circuit, and the portions out of contact with paper such as the supporting device and the connection parts are also warmed uniformly. Thus, the amount of heat diffused uselessly into the atmosphere is large and energy is much wasted, prolonging a warming-up time period since a time when an electric current is applied to the halogen lamp till a time when the heating roller reaches a developer fixing temperature (about 150° C.).
On the other hand, a fixing device having an electromagnetic induction heater as a source for heating a heating roller is available to shorten the warning-up time.
JP-A-2002-072755 (p. 4–5, FIG. 1) discloses a fixing device having an electromagnetic induction heater as a heating source. The electromagnetic induction heater is placed on the opposite side to a contact part of the heating roller with a recording material. In the electromagnetic induction heater, a coil shaped like a spiral so as to extend in the roller axial direction and made flat (flat coil) is bent and placed so as to cover like a circular arc with a constant gap maintained along the roughly semi-circumferential face of the heating roller. When an alternating current is supplied to the coil, an AC magnetic field is applied to the heating roller for heating.
However, according to the fixing device disclosed in JP-A-2002-072755, the coil is disposed so as to cover via the gap along the roughly semi-circumferential face of the heating roller. If the gap varies, the heating efficiency of the electromagnetic induction heater changes and the surface temperature of the heating roller easily varies.
Since the coil is disposed so as to be opposed directly to the heating roller via the gap, the radiant heat from the heated heating roller to the outside is transferred directly to the coil. When the coil is heated and electric resistance thereof increases, the heating efficiency of the electromagnetic induction heater is decreased.
JP-A-58-035568 (p. 2–3, FIGS. 1–3) discloses a fixing device having an electromagnetic induction heater as a heating source including a heating roller formed with a high-heat electric conductive thin layer on the surface formed like a hollow cylinder, a frame-like core made of a magnetic-permeability material, and a coil wound around one side of the frame-like core. The one side of the core around which the coil is wound is disposed with a gap between the side and the outer peripheral surface of the heating roller along the axial direction of the heating roller, and an opposite side opposed to the one side of the core is made to pierce the inside of the heating roller. An alternating current is allowed to flow into the coil to thereby introduce a magnetic flux into the opposite side of the core piercing the inside of the heating roller and the magnetic flux in the heating roller causes an eddy current to be generated on the surface of the heating roller, heating the high-heat conductive thin layer on the surface of the heating roller. Drive means of the fixing device is configured such that an end part of a pressurization roller is fixed to a sprocket connected to a rotation shaft of a motor by a chain. The rotation force of the motor is transferred to the pressurization roller and the outer peripheral surface of the heating roller and the outer peripheral surface of the pressurization roller are brought into contact with each other, so that the rotation force of the pressurization roller is transferred to the heating roller. JP-A-2002-008845 (p. 5–6, FIGS. 1–2) discloses a fixing device having an electromagnetic induction heater as a heating source including a tubular belt guide, an electromagnetic inductive belt fitted loosely onto the surface of the belt guide for rotation, and a magnetic core and a magnetic coil which are stored in the belt guide. An alternating current is applied to the coil to thereby introduce an alternating magnetic flux into the magnetic core and causes an eddy current to be generated on the electromagnetic inductive belt, heating the electromagnetic inductive belt. A rotation shaft of a pressurization roller is projected from a side wall of a chassis covering the surroundings of the electromagnetic induction heater, a heating roller and the pressurization roller. The shaft is joined to drive means outside the chassis through a gear, the pressurization roller is rotated, and a fixing belt is turned by the frictional force of the outer faces of the pressurization roller and the fixing belt.
Improvement of heating efficiency and miniaturization, space saving, etc., are required for the fixing device, and the drive means of the fixing device having high drive accuracy without impairing the heating efficiency is required.
However, according to the fixing device disclosed in JP-A-58-035568, an alternating current flows into the coil disposed outside the heating roller to introduce a magnetic flux into the core (one side of the core shaped like a frame) in the heating roller and the magnetic flux flowing into the core in the heating roller heats the surface of the heating roller. Thus, as compared with a fixing device for directly heating the surface of a heating roller by a magnetic flux of a coil disposed along the axial direction of the heating roller, the magnetic flux covering the surface of the heating roller is smaller and the heating efficiency on the surface of the heating roller is lower. When the number of windings of the coil is increased to provide a predetermined heating amount on the surface of the heating roller, the length of the coil in the axial direction thereof grows and it becomes difficult to save space in the length direction.
Since the coil is disposed so as to be opposed directly to the heating roller via the gap, the radiant heat from the heated heating roller to the outside is transferred directly to the coil. When the coil is heated and electric resistance increases, the heating efficiency of the electromagnetic induction heater is decreased.
According to the fixing device disclosed in JP-A-2002-008845, since the coil is stored in the tubular belt guide, it is difficult to radiate heat transferred from the electromagnetic inductive belt to the coil or heat from self-heating of the coil, etc. When the coil is heated and electric resistance is increased, the heating efficiency is decreased, and the temperature on the surface of the electromagnetic inductive belt varies.
JP-A-2000-214702 (p. 3–4, FIG. 1) discloses a fixing device having an electromagnetic induction heater as a heating source. The electromagnetic induction heater is placed on the opposite side to a contact part of a heating roller with a recording material. The electromagnetic induction heater has a coil wound around a magnetic substance core along the length direction of the heating roller, and gives an AC magnetic field to the coil, thereby heating the heating roller.
The heating roller includes an iron-made core metal cylinder as a core material, a heat insulating layer covering the outer periphery of the core material, and an electromagnetic induction heat generation layer covering the outer periphery of the heat insulating layer and formed by conducting an electroforming work on a metal such as nickel.
However, according to the fixing device disclosed in JP-A-2000-214702, the electromagnetic induction heat generation layer of the heating roller is formed by electroforming work and thus the manufacturing cost is increased and productivity is impaired. Since the iron-made core metal cylinder is included as a support, the core metal cylinder receives the AC magnetic field of a coil and generates heat and there is a possibility of impairing the heating efficiency heating the heating roller surface.